Various passive and active solutions have been developed to achieve a specified power quality for aircraft systems. Some passive solutions include passive filters, multi-pulse transformers, and multi-pulse autotransformers. Some active solutions include active rectifiers, active filters, and matrix converters. Multi-pulse autotransformers are generally more desirable than multi-pulse transformers in aircraft applications because of their smaller size, weight and costs. Multi-pulse autotransformers can be configured as 12-pulse, 18-pulse, 24-pulse, and 30-pulse rectification systems, and even more for aircraft applications. When 6-pulse rectifiers are paralleled, interphase transformers are normally used to achieve current sharing between the bridges, otherwise each bridge is required to carry the full current. It is also possible to achieve multi-pulse rectification systems whereby the conduction angle of diodes is not limited to 120 degrees. The advantage of this type is that they do not need to use interphase transformers. For example, a 40-degree conduction of diodes can be achieved by using a properly configured 18-pulse autotransformer. Similarly, a 30-degree conduction of diodes can be achieved by using a properly configured 12-pulse autotransformer. These solutions can be used with alternating current (AC) power systems that operate with constant voltage constant frequency (CVCF) or constant voltage variable frequency (CVVF) to provide power to aircraft systems. The CVCF systems typically operate at 400 Hz for commercial aircrafts.
Emissions from these AC power systems may produce radiated and conducted electromagnetic interference (EMI) for aircraft systems. For example, the emissions may transmit EMI distortion back into the AC power systems. Therefore, maintaining low distortion is important for the proper operation of devices connected to the AC power systems. EMI standards related to aircrafts vary but may include Mil-Std-461E, RTCA/DO160D, and IEC 61000-4-3. For this reason, as well as others, EMI requirements are placed on AC power systems by aircraft manufacturers. Similarly, power quality standards are placed on AC power systems by aircraft manufacturers to lower the AC current harmonics withdrawn by the loads.
Thus, it should be appreciated that a need exists for an EMI filter system that reduces interference in aircraft applications. The invention fulfills this need as well as others.